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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 12 January 2016 by dana1981

Climate sensitivity – the amount of global surface warming we’ll see as a result of rising atmospheric carbon dioxide levels – has become contrarians’ favorite basis to argue against cutting carbon pollution. If the Earth’s climate is relatively insensitive to rising carbon levels, then it’s somewhat less urgent that we stop burning massive quantities of fossil fuels. However, a new NASA study indicates that’s not the case.

There are a few different ways that climate scientists estimate the Earth’s sensitivity to rising carbon. When they look at climate changes in the distant past (paleoclimate), and at simulations from complex climate models, they get about the same result: if the amount of carbon dioxide in the atmosphere doubles, temperatures will rise between 2°C and 4.5°C, most likely 3°C.

However, a few studies in recent years using a third method have yieldedsomewhat lower results. This method uses recent measurements of temperature and heat changes, combined with estimates of how “forcings” like the increased greenhouse effect have caused the Earth’s energy balance to change, all input into somewhat simpler climate models.

These results caused the latest IPCC report to drop its lower estimate of the likely climate sensitivity to double carbon dioxide from 2°C to 1.5°C. Climate scientists were faced with the question, why did this third approach (known as the “energy budget approach”) yield somewhat lower results than others, and which estimate is right?

A new study by Kate Marvel, Gavin Schmidt, Ron Miller, and Larissa Nazarenko at the NASA Goddard Institute for Space Studies appears to have found the answer. They drew upon previous research by Drew Shindell and Kummer & Dessler, who identified a flaw in studies taking the energy budget approach. Those studies had assumed that the Earth’s climate is equally sensitive to all forcings.

In reality, as world-renowned climate scientist James Hansen noted in a 1997 paper, some forcings are more efficient at causing the Earth’s surface temperature to change than others. Those in which the effects are focused in the northern hemisphere tend to be more efficient, for example. Andrew Dessler explains in the video below.

Drew Shindell first identified this deficiency with these low sensitivity studies in a 2014 paper. Shortly thereafter, Kummer & Dessler published a paper noting that this issue could potentially bring the climate sensitivity estimates from the energy budget method in line with estimates from climate models and paleoclimate studies. This new NASA paper builds upon those previous studies by better quantifying the efficiencies of different forcings over the historical period and the effect this has on energy budget approach climate sensitivity estimates.

The NASA scientists ran climate models using just one forcing at a time – changes in greenhouse gases, aerosol pollution, land use changes, etc. – to see how efficient each is at changing the global surface temperature. As it turns out, forcings that have tended to cause cooling, like increased aerosol pollution, are particularly efficient.

The scientists then repeated the energy budget study approaches incorporating what they learned about the various forcing efficiencies, and found that the previous climate sensitivity estimates were indeed biased low. As the NASA study authors wrote,

Climate sensitivities estimated from recent observations will therefore be biased low in comparison with CO2-only simulations owing to an accident of history

The “accident of history” is that the more efficient forcings happen to be those that have had a cooling effect on temperatures in recent decades, while the less efficient forcings happen to be those that have caused warming. By assuming they were all equally efficient, the previous energy budget studies, for exampleby Nic Lewis and Judith Curry, biased their climate sensitivity estimates low. The new best estimate puts climate sensitivity right around 3°C warming in response to doubled carbon dioxide levels, in line with estimates from climate models and paleoclimate studies.

It’s also important to note that relying solely on the previous energy budget model results is a clear-cut case of cherry picking. That method was the outlier, yielding lower climate sensitivity estimates than other approaches. In fact, there have been a number of recent studies using observed changes in cloud cover, finding that the climate models with the higher sensitivities are those that reproduce the cloud changes most accurately.

Comments

doi: 10.1038/ncomms10266 is a nice paper on the effects of clouds in Greenland preventing refreeze and ratcheting up melt, especially since firn is full. The paper illustrates that clouds have a spatial, temporal, ocean and many other fingerprint, and averaging these effects would mislead.

This is a critical point in the debate. Science works by refuting false hypotheses. The contrarians' hypothesis is that "Human additions to atmospheric CO2 will not adversely affect the climate". Low climate sensitivity is absolutely crucial to their case. They were relying on Lewis and the 20th century TCR studies to sustain low climate sensitivity, and since the studies have been shown to be flawed, it is surely time for us to say loud and clear that the contrarian hypothesis has no merit.